INDICES
The three indices that are depicted below are not absolute measures of juvenile abundance of alewife, blueback herring and American shad. Instead, each record is believed to be a surrogate of population size that
is best interpreted as a relative measure of recruitment in a time series of data - in this case, since 1979. Often, an historical record of strong and weak years of recruitment is reflected as a discernible pattern in the
age composition of adult populations. Thus, a strong recruitment of American shad in 1996 might produce a very abundant year class when that cohort appears in the fishery 4-5 years later. In this way, indices that
are particularly sensitive to changes in young-of-the-year abundance can be applied to predictions of future year-class strength. One goal of this research program is to determine if these historical records are
applicable to problems of fisheries management.Juvenile Blueback Herring Index (1979-1997)